Various strategies are being pursued to minimize the production and/or release of undesirable emissions from combustion processes. One such strategy is the development of technologies for the specific removal of acid gases from gas mixtures, such as the exhausts of carbon combustion processes. The separation of acid gases, such as CO2, from gas mixtures has been carried out industrially for over a hundred years, although no known process has been used on a large scale such as that required by large, industrial power plants. Of the numerous processes used for CO2 separation, current technology mainly focuses on the use of various solvents, such as alkali carbonates in the BENFIELD™ Process (UOP, LLC), alcoholamines in the ECONAMINE FG PLUS™ process (Fluor Corporation), and alcohols, diols, and ethers in the RECTISOL® process (Lurgi, GMBH) and the SELEXOL™ solvent (The Dow Chemical Company). In a typical solvent-based process, the gas mixture to be treated is passed through a liquid solvent that interacts with acidic compounds in the gas stream (e.g., CO2 and SO2) and separates them from non-acidic components. The liquid becomes rich in the acid-gas components, which are then removed under a different set of operating conditions so that the solvent can be recycled for additional acid-gas removal.
Other strategies for acid gas removal employ solid-phase sorbents. In some applications, solid phase sorbents are desirable, as they can provide various benefits, including typically requiring lower heat for regeneration. Solid phase sorbents generally employ porous materials (e.g., mesoporous silicates, zeolites, metal-organic-frameworks, and the like) that can selectively isolate acid gases such as CO2. Addition of specific functional sites, such as amine sites, within such materials represents a means to increase the selectivity and absorption capacity of such solid phase sorbents. One such material can be obtained by associating a solid, porous material with an amine-containing polymer, such as polyethyleneimine (PEI).
PEI has been previously investigated for acid gas absorption and branched PEI in particular has a high theoretical capacity for acid gas absorption due to the number of primary and secondary amines in its molecular structure. For example, Wang et al. have prepared amine-modified materials via physical impregnation and chemical graft of polyethyleneimine (PEI) on different porous materials (see D. X. Wang et al., Ind. Eng. Chem. Res. 51(2012): 3018-3057); Xu et al. have described a PEI-modified mesoporous molecular sieve of MCM-41 type (MCM-41-PEI) (see X. C. Xu et al., Energy Fuels, 16 (2002): 1463-1469); Hicks et al. have studied a covalently tethered hyperbranched aminosilica (HAS) material (see J. C. Hicks et al., J. Am. Chem. Soc. 130 (2008): 2902-2903); Drese et al. have disclosed hyperbranched aminosilica (HAS) adsorbents prepared via the ring opening polymerization of aziridine in the presence of a mesoporous silica SBA-15 support (see J. H. Drese, Adv. Funct. Mater. 19 (2009): 3821-3232 and Microporous Mesoporous Mater. 151 (2012): 231-240). Many of the PEI-based solid sorbents prepared to date are not cost-effective to prepare and/or result in release/leaching of some percentage of the PEI in use.
One further challenge with the use of PEI is that it is highly soluble in water. Where gas streams to be treated are humid (i.e., comprise some degree of water), unattached PEI can readily leach from the porous material with which it is associated. Similarly, PEI that is tethered to a porous material may also leach from the porous material due to reaction between the water and the linkages between the PEI and the porous material, cleaving the PEI from the porous material. Accordingly, such strategies can lead to significant loss of effectiveness of the sorbent. It would be advantageous to formulate a solid phase sorbent capable of effectively removing acid gases from gas streams (particularly humid gas streams) over extended periods of use, regeneration, and reuse.